1. Field of the Invention
The present invention relates generally to seismic surveying of subterranean geological formations. More particularly, the present invention relates to improved seismic sensors for monitoring seismic waves at a location within a liquid-filled borehole, and methods for their use.
2. State of the Art
Seismic surveying is used, by way of example, to examine subterranean geological formations for the potential presence of reserves of hydrocarbons such as petroleum, natural gas and combinations thereof as well as the extent or volume of such reserves. Seismic waves, also termed acoustic waves, are emitted from a seismic energy source to penetrate through layers of rock and earth, and under certain conditions are reflected and refracted by variations in the composition of the subterranean formations in the path of the waves. Seismic sensors configured as motion sensors in the form of geophones or accelerometers or pressure sensors in the form of hydrophones receive the reflected and refracted waves and convert them into corresponding electrical signals, which are then analyzed for the presence and extent of formations containing oil and gas deposits.
An increasingly common technique for subterranean exploration is known as borehole seismic surveying, wherein one or more seismic sensors are placed below the earth's surface in the liquid-filled borehole of a well. The seismic energy source may be located above ground, or may also be placed in a borehole to emit seismic waves within close proximity to the area of interest. By recording the seismic waves at various depths below the surface, a profile is acquired that provides more detailed information about the surrounding area than may be acquired using surface-based seismic sensors. These higher resolution views of subterranean formations can thus be examined for the presence of hydrocarbon reserves that might otherwise remain hidden.
In order to reduce the time required for data acquisition, an array of seismic sensor modules is deployed in the borehole to take simultaneous readings at different locations along its length. The sensor modules, typically in the form of sondes containing geophones, are lowered into the borehole on an elongated structure including a conductive cable such as a wireline, tubing string or other suitable structure. The geophones are configured for measuring the seismic waves in three directions or axes, to give a reading for each of the orthogonal components of the waves. For optimum sensing by the geophones, it is necessary that there be a good interface between the sondes and the subterranean formation volume surrounding the borehole to ensure effective transmission of seismic energy. In the prior art this has often been accomplished by using extendable mechanical arms that urge the sondes into firm contact with the borehole wall, and provide an improved mechanical coupling for conducting waves to the geophones. In boreholes that are lined with metallic casings, magnetic means have also been used in an attempt to couple sondes to the borehole wall. All of the foregoing types of systems are controlled from above the surface to deploy the interface structures for the geophones, and involve complicated mechanisms for extending and retracting arms or orienting and activating magnets. Limitations on transmitting electric and hydraulic power to significant depths are another significant concern. The prior art approaches result in increased equipment cost and enhanced possibility of a malfunction causing the sondes to become stuck within the borehole and requiring an expensive retrieval, or “fishing,” operation. Further, wave components traveling perpendicular to the borehole, versus wave components traveling up and down the borehole liquid column, are subject to different influences on their propagation. Interfacing all of the sondes in the same wall-coupling manner may not improve geophone readings for all three x, y and z sensor directions.
What is needed, therefore, are robust and uncomplicated methods and apparatus that achieve an improved interface between seismic waves and sensor modules within a borehole to provide high-resolution seismic survey data, while overcoming the problems associated with the prior art.